二元醇混合物汽液相平衡的GEMC模拟与微观结构分析

GEMC simulation and microstructure analysis of vapor-liquid equilibrium for diol mixtures

采用Gibbs蒙特卡罗模拟方法和OPLS-AA力场计算了乙二醇（EG）/1,2-丁二醇（1,2-BDO）、乙二醇/1,3-丁二醇（1,3-BDO）、乙二醇/1,4-丁二醇（1,4-BDO）三组二元醇混合物的汽液相平衡数据,通过与实验数据及Wilson状态方程计算数据的比较验证了方法和力场对体系汽液相平衡预测的适用性。对混合体系饱和液相的径向分布函数分析可知,各二醇间存在与羟基官能团位置近似无关的强氢键作用和作用范围不同的范德华作用。经体系O-O平均数目的统计及氢键缔合比例分析发现,随乙二醇组成增大,3种体系的微观结构由主要包含较大的BDO-BDO、BDO-EG缔合结构（〉6）向包含较小的EG-BDO、EG-EG缔合结构（2~4）转变,且缔合比例下降。Molclus＋Gaussian09对小分子EG、1,2-BDO缔合结构进行能量优化与构型搜索,由此提出了二醇缔合比例随EG组成变化的可能原因：1,2-BDO间缔合结构稳定于其他缔合结构,随缔合分子数增大,各缔合结构稳定性增加,但EG自缔成大分子结构的稳定性与其和BDO缔合成小分子结构稳定性差异减小。

Gibbs ensemble Monte Carlo simulation（GEMC）method is used to investigate the vapor-liquid equilibria and microstructure of ethylene glycol（EG）/1,2-butanediol（1,2-BDO）,EG/1,3-but-anediol（1,3-BDO） and EG/1,4-butanediol（1,4-BDO） mixtures. The simulation approach and OPLS-AA force field are proved to be adaptable to calculate the vapor-liquid equilibria by comparing the simulating results with experimental data or results calculated from Wilson EOS. Analyses of radial distribution functions for the mixtures above show that there is strong hydrogen-bonding interaction that is unrelated to the site of hydroxyl and Van der Waals interaction occurred in different regions among diol-mixtures. Statistics of the number of O-O atom pair and analyses of the hydrogen-bonding association structure reveal that with increasing EG in the mixture,the microstructure of the saturated liquid phase changes from the state consisting of many big BDO-BDO and EG-BDO hydrogen-bonding association structure（〉6）to that composed of many small EG-EG and EGBDO hydrogen-bonding association structure（2~4）with decreased proportion of associated molecules. Molclus and Gaussian09 are utilized to search for the stable structure of the EG,1,2-BDO small hydrogen-bonding configuration. Then,one possible reason is proposed to explain why the proportion of diols association structure changes in some law above. That is,1,2-BDO-1,2-BDO association structure behaves more stable than others,and,as the number of associated molecules grows,the associated structure changes to be more stable,while the difference between the stability of big EG-EG association structures and smaller EG-1,2-BDO association structures decreases.